As is known, in the field of photon-detection techniques, there is a need for devices that enable detection of electromagnetic radiation with high sensitivity, which are thus able to detect an even limited number of photons associated with the electromagnetic radiation.
To this end, so-called avalanche photodiodes (APDs) have been known for some time and typically comprise a junction of P-N type or P-I-N type, an anode contact, and a cathode contact, capable of biasing the junction. In particular, an APD is biased by applying, by an appropriate biasing circuit, a reverse biasing voltage VA that is very high, but lower (in modulus) than the breakdown voltage VB of the junction in the APD, thus biasing the junction in the linear region. Given the high reverse biasing voltage VA, the junction has a wide depletion region, subject to a high electric field.
Operatively, given a photon that impinges on an APD, the photon traverses the junction, and in particular traverses the depletion region, where it can be absorbed, generating a corresponding electron-hole pair. If the photon is absorbed, the electron and the hole thus created, referred to, as a whole, as “carriers”, are accelerated by the electric field in opposite directions, with consequent generation of a current indicating the absorption having occurred. Since the APD is biased in the linear region, the current is proportional to the number of photons absorbed, thus to the number of photons that impinge on the APD. In addition, given the high electric field, the acceleration is such that the carriers acquire kinetic energies sufficient to cause generation of further electron-hole pairs by atomic ionization by impact, i.e., irrespective of the absorption of photons. Each carrier created after absorption of a single photon may thus generate one or more electron-hole pairs, with consequent multiplication of the absorption current and generation of an avalanche current. The avalanche current is appropriately collected, for example, through external circuitry, at the anode and cathode contacts, and represents an output signal of the APD.
It is noted that, to be precise, the reverse biasing voltage VA applied to the APD coincides with the effective voltage Ve present across the junction only in the absence of photons. In fact, in the presence of photons, and thus of current generated within the APD, the effective voltage Ve across the junction may be lower, in modulus, than the reverse biasing voltage VA, as described hereinafter in greater detail. However, in the present document it is assumed, except where otherwise explicitly specified, that the effective voltage Ve across the junction coincides with the reverse biasing voltage VA.
The most widely available APDs comprise a P-I-N junction, capable of guaranteeing a depletion region that is wider than the one obtainable with a P-N junction, and moreover comprise an additional region of semiconductor material, which is slightly doped, either of P- or N-type. This additional region is arranged in such a way that the electric field inside it is particularly high so as to favor the process of atomic ionization by impact.
In detail, the APD comprises a structure formed by semiconductor regions that follow one another according to a P-N-I-N scheme, or else a N-P-I-P scheme, the additional region being thus arranged between the intrinsic region I, which is at most only slightly doped, and the region of the P-I-N junction having a conductivity of an opposite type with respect to the conductivity of the additional region. On the basis of the physical phenomena that occur therein, the additional region is also known as “multiplication area” in so far as it is the site of the phenomena of ionization by impact, while the intrinsic region is also known as “absorption area”, since the majority of the photons are absorbed therein.
APD photodetectors of the type described, also known as “separate absorption and multiplication avalanche photodiodes” (SAM APDs), make it possible to achieve gains, i.e., amplifications of the current generated by absorption of a photon, of the order of 102. However, on account of noise and of noise amplification, APD photodetectors do not enable detection of individual photons.
In order to improve responsivity further, so-called “Geiger-mode avalanche photodiodes” have been proposed, which are theoretically able to detect individual photons.
A Geiger-mode avalanche photodiode, also known as single-photon avalanche diode (SPAD), is biased by applying a reverse biasing voltage VA higher in amplitude than the breakdown voltage VB, typically higher by 10-20%, so that a single electron-hole pair, generated by absorption of a photon, is sufficient to trigger a ionization process that causes an avalanche multiplication of the carriers, with gains of around 106 and consequent generation in short times (hundreds of picoseconds) of the avalanche current.
The gain and the likelihood of detection of a photon, i.e., the responsivity of the SPAD, are directly proportional to the reverse biasing voltage VA applied to the SPAD. In fact, the more the reverse biasing voltage VA exceeds the amplitude of the breakdown voltage, the higher the likelihood of occurrence of an avalanche generation of charge carriers.
However, high reverse biasing voltages VA mean that, even in the absence of incident photons (dark conditions), a single charge carrier, generated, for example, by transfer of thermal energy, is sufficient to trigger the avalanche-ionization process, generating the so-called “dark current”, which interferes negatively with normal use of the SPAD.
In addition, the fact that the reverse biasing voltage VA is appreciably higher than the breakdown voltage VB causes the avalanche-ionization process, once triggered, to be self-supporting. Consequently, once triggered, the SPAD is no longer able to detect photons, with the consequence that, in the absence of appropriate remedies, the SPADs described manage to detect arrival of a first photon, but not arrival of subsequent photons.
In order to be able to also detect these subsequent photons, it is necessary to quench the avalanche current generated within the SPAD, stopping the avalanche-ionization process. In detail, it is necessary to reduce, for a period of time known as “hold-off time”, the effective voltage Ve across the junction so as to inhibit the ionization process and quench the avalanche current, as described hereinafter. Subsequently, the initial junction-biasing conditions are reset so that the SPAD is again able to detect photons. Since during the hold-off time the SPAD is not able to detect photons, it is desirable for the hold-off time to be as short as possible.
In order to reduce the effective voltage Ve across the junction after absorption of a photon, the SPADs adopt so-called “quenching circuits”.
Among others, quenching circuits of a passive type are known in the art, comprising a quenching resistor arranged in series with the junction and having a resistance of the order of hundreds of kiloohms.
In the absence of photons, the presence of the quenching resistor does not alter the effective voltage Ve across the junction, which is equal to the reverse biasing voltage VA. However, after absorption of a photon and consequent triggering of the ionization process, the avalanche current that is triggered thereby, by flowing in the quenching resistor, causes an exponential reduction in the amplitude of the effective voltage Ve across the junction, which drops until it is slightly higher than the breakdown voltage VB. As regards, instead, the avalanche current generated within the SPAD, it passes from a zero value to a peak value substantially immediately after triggering of the avalanche-ionization process (turning-on of the SPAD), then decreases exponentially towards an asymptotic value, inversely proportional to the resistance of the quenching resistor and directly proportional to the difference between the reverse biasing voltage VA applied of the SPAD and the breakdown voltage VB, said difference being generally known as “overvoltage” (OV).
In detail, it is possible to show that the avalanche-ionization process is stopped if the value of the avalanche current drops below a threshold value known as “latching current” IL. Consequently, a quenching resistor having a resistance equal to Rq is able to quench the avalanche current for an overvoltage equal at most to the product Rq·IL, i.e., for a reverse biasing voltages VA not higher than VB+Rq·IL. If these conditions are respected, the avalanche current is quenched. Then, the SPAD, the behavior whereof is to a certain extent comparable to that of a capacitor, recharges exponentially through the quenching resistor so that the effective voltage Ve across the junction returns to being equal to the reverse biasing voltage VA, the SPAD being thus ready to detect the arrival of a new photon. During the recharging time, i.e., in the time interval in which the effective voltage Ve increases exponentially until it returns equal to the reverse biasing voltage VA, the SPAD has a reduced responsivity, which increases as the effective voltage Ve increases.
On the basis of the above, it may be inferred that applications that are particularly demanding in terms of responsivity typically require high overvoltages OV, thus high reverse biasing voltages VA, with the consequence that the quenching resistor has a high value; otherwise, it might be impossible to quench the avalanche current and, thus, to detect more photons.
High quenching resistance entails a longer recharging time than lower resistance. On the other hand, it leads to an additional advantage. In fact, during the recharging time the SPAD is less sensitive to the arrival of the photons, but is also less sensitive to spurious pulses (“afterpulsing”), which usually degrade the performance of the SPADs. In detail, afterpulsing consists of the secondary emission of carriers due to lattice defects in the depletion region, which create intermediate energy levels (comprised between the conduction band and the valence band) that are able to capture one or more carriers of the avalanche current, then releasing them with unpredictable delays, causing an increase in the dark current and distorting the output signal of the SPAD.
On the basis of the above, the quenching resistors are sized in the design stage according to the reverse biasing voltage VA to be applied to the SPAD, which in turn depends upon the application intended for the SPAD, and in particular upon the responsivity and upon the quantum efficiency required. In addition, the resistance of the quenching resistor is sized as a function of the recharging times and of the sensitivity to afterpulsing specified by the application for which the SPAD is devised. For example, where the rate of arrival of the photons is predictably low, as, for example, in astrophysical applications, the quenching resistor is sized with a high resistance so as to limit the afterpulsing, the consequent increase in the recharging time not being particularly important in this type of application.
SPADs of a known type may thus be optimized for a specific application; consequently, their performance may fall off markedly when they are used in conditions that are not perfectly in compliance with the design conditions.
Similar considerations may be made for so-called SPAD arrays, and moreover for so-called silicon photomultipliers (SiPMs), used in order to improve the performance that may be obtained with individual SPADs.
In particular, an array of SPADs, two examples whereof are shown in Italian Patent Application Nos. TO2008A000046 and TO2008A000045 filed on Jan. 18, 2008, and which are incorporated by reference, is simply formed by a planar matrix of SPADs grown on a same substrate.
A SiPM is a particular array of SPAD. In detail, the SiPM is formed by a matrix of SPADs grown on a same substrate and provided with respective quenching resistors, of a vertical type and integrated in the SPADs, the quenching resistors being uncoupled from, and independent of, one another. In addition, the anode and cathode contacts of each SPAD are configured to be coupled to a single voltage source. Consequently, the SPADs of the SiPM can be biased at a same reverse biasing voltage VA. In addition, the avalanche currents generated inside them are multiplied together so as to generate an output signal of the SiPM equal to the sum of the output signals of the SPADs. As regards the terminology, in the technical field of SiPMs it is common to refer to the ensemble photodiode-quenching resistor as “pixel”, the SiPM being thus formed by an array of pixels.
The SiPM is thus a device with a large area and high gain, capable of supplying, on average, an electric output signal (current) proportional to the number of photons that impinge on the SiPM. In fact, since the quenching resistors are uncoupled from one another, each photodiode of the SiPM behaves as an independent binary counter, while the output signal of the SiPM is proportional to the number of pixels activated, i.e., to the number of SPADs in which the avalanche-ionization process (detection of a photon) has been triggered, this number being in turn proportional to the number of incident photons.
As has been indicated, notwithstanding the better performance provided by SiPMs, these also may undergo improvement as to versatility of use.